Galactic Pile-Up May Point to Mysterious New Dark Force in the Universe

LONG BEACH, California – By closely mapping the mass of an enormous galactic collision, astronomers may have uncovered a type of force that only affects dark matter.

The results come from observations of the Musket Ball Cluster, a vast celestial object located about 5.23 billion light-years away in the constellation Cancer. Galaxies are usually gravitationally bound to other galaxies, creating massive galactic clusters. The Musket Ball Cluster is an example of what happens when two such galactic clusters – each composed of hundreds of individual galaxies – crash into one another.

Scientists know the visible stars in these galaxies make up only about 2 percent of the total mass in the cluster. About 12 percent of the mass is found in hot gas, which shines in X-ray wavelengths, while the remaining roughly 86 percent is made of invisible dark matter. Because the galaxies make up so little of the mass of the system and the spaces between them are so large, they don't really do much of the crashing. Odds are that they will simply sail by one another as the clusters merge. It's mostly the gas that collides, causing it to slow down and fall behind the galaxies.

LRG 3-757, an example of an Einstein ring.

ESA/Hubble & NASA

The dark matter is mapped using a quirk in Einstein’s theories of gravity. According to General Relativity, the gravitational fields of massive objects like galaxies bend light. If there is a large galaxy in the way of a distant light source, observers on Earth will see that light distorted, often into a ring-like shape, like the Hubble image at left. By looking at how light from a distant object is bent by the Musket Ball Cluster, scientists can infer where the dark matter is.

But when astronomers did this with high precision, they discovered something odd: The dark matter clumps were slowing down relative to the visible galaxies in the cluster.

“We see this offset between the dark matter and the galaxies of about 19,000 light-years,” said astronomer William Dawson of the University of California, Davis, who presented his team's result during a talk Jan. 7 here at the American Astronomical Society 2013 meeting.

The reason this is strange is that dark matter is thought to barely interact with itself. The dark matter should just coast through itself and move at the same speed as the hardly interacting galaxies. Instead, it looks like the dark matter is crashing into something – perhaps itself – and slowing down faster than the galaxies are. But this would require the dark matter to be able to interact with itself in a completely new an unexpected way, a “dark force” that affects only dark matter. This would be a new fundamental force of the universe, in addition to the four known forces: gravity, electromagnetism, and the strong and weak forces.

Such a force has been speculated theoretically in previous work and even searched for in small colliders but, if Dawson’s results turn out to be true, this would be the first observational evidence of its existence. Though the dark force is not part of any current model of physics, it could help explain certain behavior seen in dark matter.

In particular it would help solve the core/cusp problem, an outstanding mystery seen in dwarf galaxies and star clusters. If dark matter only feels the force of gravity, it should tend to clump in the center of these objects. But astronomers over and over observe the opposite: The dark matter in dwarf galaxies and star clusters is evenly distributed. If dark matter can interact through some sort of dark force, it can bump into itself and puff out, like a hot gas.

The finding could help open up observations of the so-called “dark sector,” a hypothetical set of forces and particles that don’t affect our own ordinary matter. Though dark matter models tend to assume the particles are simple and have no extra forces, there’s no particular reason this should be. Dawson suggested imagining some alien, scientific beings composed entirely of dark matter, who might not even consider that our version of matter has so many complex forces and interactions because they can't detect them.

While agreeing that the results are neat and have a potentially huge payoff, astronomer Douglas Finkbeiner of Harvard, who was not involved in the work, isn't completely convinced by them yet. "It is good to remember that every such hint of exotic dark matter particle properties has always been wrong," he wrote in an email to Wired.

Finkbeiner should know. In 2008, he was part of research team that thought it had glimpsed a signal of a dark force in data from the PAMELA satellite. The results ended up being discounted a few years later.

Dawson knows his findings are preliminary, and even he is fairly skeptical of the dark force interpretation. His team can say with roughly 85 percent confidence that what they are observing is due to dark matter interacting with itself.

“Those are good odds in Las Vegas, but as scientists we can’t make grand claims with there still being a 15 or 20 percent possibility of this being noise in the measurement,” he said. The bending of light by massive objects is very tricky to observe, and it could turn out there is some problem in the team's measurements.

For now, Dawson is working with his collaborators to analyze data from other massive galaxy cluster collisions and also discover new ones. If they see the same results on these systems, it would bolster the idea of a possible dark force. Otherwise, it will mean that dark matter is fairly simple and scientists need other explanations for the core/cusp problem.

“We need observations to either reign in the theoretical musings or motivate people to think harder about their dark matter models,” said Dawson.